Precast tilt-up, cast on site or off site, (also known as precast tilt-slab or tilt-wall) concrete construction is not new; it has been in use since the turn of the century. Since the mid-1940s it has developed into the preferred method of construction for many types of buildings and structures in the U.S. Precast concrete construction has many advantages that are well know in the art. The precast concrete panels can significantly reduce the initial cost of construction, increase the life of the structure and provide a relatively low-cost, low-maintenance building envelope. Depending on the size and type of application, such precast panels can be fabricated and stored offsite then delivered just in time for erection and installation. They can also be made on the construction site thereby eliminating relatively expensive transportation costs.
After concrete footings and a concrete slab have been poured and properly cured, a precast tilt-up concrete structural panel can be formed on the concrete slab. In tilt-up concrete construction, vertical concrete elements, such as walls, columns, structural supports, and the like, are formed horizontally on a concrete slab; usually the building floor, but sometimes on a temporary concrete casting surface near the building footprint. After the concrete has cured, the elements are tilted from horizontal to vertical with a crane and braced into position until the remaining building structural components are secured. In the same way the precast concrete panels can be formed in an offsite location using various types of forms well known in the art. After curing the precast and cured panels are transported to the building site and erected by means and methods well known in the art.
Construction of a precast concrete wall panel is begun by carefully planning out the size and shape of the wall panel on a suitable surface, such as the concrete slab (i.e., floor) of the building being constructed. Wooden concrete forms, usually made from 1× or 2× lumbar, are constructed on the perimeter of the proposed concrete wall. Typically, the wall panel depth (i.e., thickness) is designed to fit the depth of standard dimension lumbar, such as 5½-inch or 7¼-inch thick structural panels. Form sides are supported and secured to the concrete slab by wood or steel angle supports. Door and/or window openings can be formed after the perimeter framing is completed. A form release agent and bond breaker is then applied to the concrete slab and to panel forms in accordance with manufacturer recommendations.
After the form is constructed, a grid of steel rebar is constructed and tied in-place within the form to reinforce the structural panel. Plastic or metal support chairs are used to support the rebar grid at a proper depth. Embeds and inserts can be attached to the side forms or to the rebar grid. Embeds are used to attach the structural panel to footings, other panels, columns, slabs, roof systems, or attachment of building accessories. Inserts provide attachment points for lifting hardware and temporary braces.
Before concrete is placed in the form, the slab or casting surface must be cleaned and a release bond breaking agent is applied to prevent the panel from bonding to the casting surface. Regardless of the type of bond breaking agent used, there is always a certain amount of bond formed between the precast panel and the casting surface that must be broken before the panels will separate from the casting surface. Additional steel reinforcement is factored in so that the concrete panels can be lifted in place without damage. Concrete is then placed in the form in the same manner as floor slabs. The concrete is usually consolidated to ensure good flow around the steel rebar grid. Then, the concrete surface can be finished in any desired manner, such as trowel finish or other types of architectural finishes and patterns.
Since conventional precast concrete panels are exposed to the ambient temperature, the concrete temperature changes hourly and/or daily depending on the weather. These constant temperature changes cause internal stress in the curing concrete due to the expansion and contraction generated by the temperature changes. Such internal stress can cause cracking or microcracking. As a result, the life expectancy of the concrete structure is reduced. Additional steel reinforcement is often necessary to compensate for this expansion and contraction.
Precast tilt-up concrete panels have a large thermal mass exposed to ambient temperatures. They retain the heat in the summer or the cold in the winter very well. Therefore, precast tilt-up concrete buildings generally have relatively poor energy efficiency. Such buildings usually require a relatively large amount of energy to keep them warm in the winter and cool in the summer. Since most precast concrete panels are not insulated, they can receive insulation on the inside through the use of furring systems or on the outside with EIFS. More recently, new methods of insulating precast concrete panels have been employed. One of the most effective methods of insulating tilt-up concrete walls, however, is the method known as “sandwich” insulation. This method involves placing a layer of insulation between a structural concrete layer and an architectural or non-structural concrete layer during the casting of the panel and then tilting this entire composite construction as a panel. While this method improves the insulating properties of the wall and therefore the energy efficiency of the building, it has several drawbacks. Instead of having one layer of concrete, the “sandwich” creates two; one that is structural with the larger thermal mass that faces the inside of the building and is insulated from the elements. The second layer of concrete is thinner and placed on the exterior of the building; i.e., on side of the panel opposite the insulated structural layer. It is easy to see why it is more expensive and time consuming to cast concrete using this method. Also, since there is still a significant amount of concrete in the outside layer exposed to the ambient temperatures, the “sandwich” system does not perform as energy efficiently as it was expected.
Before the precast tilt-up concrete panel can be transported to the building site or erected into place, the concrete must achieve a desired minimum degree of strength. A precast tilt-up wall panel with low concrete compressive strength is more susceptible to failure by erection stresses. Therefore, it is important to know the compressive strength of the concrete at the time of erection. It is normal to have a minimum concrete compressive strength of 2,500 psi (18 MPa) before the titling operation begins; preferably 4,000 psi. For conventional Portland cement-based concrete, without additives to increase compressive strength, sufficient compressive strength is usually reached in five to seven days. However, depending on the weight of the panel being lifted, it may be necessary to change the concrete mix design to provide a stronger concrete compressive strength. Moreover, early concrete compressive strength is significantly affected by environmental conditions at the work site, especially temperature variations. In the construction industry, time is money. Thus, contractors frequently resort to the use of expensive concrete additives to make sure that the concrete has sufficient early strength to endure the stresses of erection.
The insulation of tilt-up concrete panels has not been dealt with extensively. In fact, few practical systems exist for insulating tilt-up concrete panels. U.S. Patent Application Publication No. 2008/0313991 discloses one system for insulating tilt-up concrete panels (the disclosure of which is incorporated herein by reference). This system uses panels of molded expanded polystyrene or extruded expended polystyrene to form the bottom surface of a horizontal mold for a tilt-up concrete panel. The foam insulating panels include dovetail-shaped grooves into which plastic concrete will flow, thereby attaching the foam insulating panels to the cured concrete panel. During the hoisting of the precast tilt-up concrete panels to a vertical position, a certain amount of deflection takes place in the panels. This deflection may cause the foam to come loose. Also, there is no mechanical attachment or reinforcement of the foam to the concrete. This system is not entirely desirable because, among others, it does not provide a system for a secure attachment of the foam to the concrete panels during hoisting or the life of the building. Also, it does not provide a system for attaching different types of exterior finishes or cladding and it does nothing to improve the physical properties of the concrete panel.
Therefore, it would be desirable to produce a precast concrete molding system for tilt-up concrete panels that allows concrete to achieve the maximum compressive strength possible in the shortest amount of time in any season and any type of weather and to be erected more quickly than prior art tilt-up concrete systems. It would also be desirable to provide a system for relatively easily and efficiently insulating tilt-up concrete panels or other structures to achieve the highest energy efficiency possible. It would also be desirable to provide an integrated precast concrete tilt-up system that provides for the installation of all types of exterior finishes or cladding systems to tilt-up insulated concrete panels.